Making a Joule Thief

The Joule Thief is a fairly well-known circuit using a single transistor and an inductor to light a white LED from a single cell (1.5V). Now, a white LED requires about 3V in order to light up, so it won't work at all when simply connected to, say, an AA battery. This simple circuit forms an oscillator that generates a slightly higher voltage from the battery, and that's sufficient to light the LED. It also has the effect of using up all the remaining energy in a battery that would otherwise be regarded as “dead”. This is because, as the battery voltage falls, the circuit continues to work, right down to a voltage well below the nominal 1.5V of a fresh cell.

I made my version using a ferrite toroid from a scrap PC power supply, a BC548 transistor and a 470Ω resistor.

Photos of my Joule Thief are available in this Flickr set.